As a capacitor in which electrolytic solution is provided between electrode bodies, an electric double layer capacitor (hereinafter also may be simply referred to as capacitor) is known for example. The electric double layer capacitor uses a large electric double layer capacity by a combination of activated carbon that has a large specific surface area of electrode material and that is electrochemically-inactive and electrolyte. The electric double layer capacitor has a characteristic that electrochemical reaction is not caused by the charge and discharge and rapid charge and discharge can be provided with high current, providing a higher power density than that of a chemical battery, for example. The electric double layer capacitor has been expected for the application to a high-current generation circuit, an instant compensation power source, and a load leveling circuit for example.
When the above capacitor is used for a long time, a dry-up mode deterioration proceeds in which the inner electrolytic solution gradually evaporates to consequently increase the internal resistance to reduce the capacitance, thus causing the termination of the service life.
One conventional method determines the deterioration of a capacitor by applying a square wave signal having a low frequency as a measurement signal to a target capacitor to integrate a predetermined part of the reply signal to detect the change of the characteristic of the capacitor based on the integration value. This deterioration determination method is disclosed by Japanese Patent Unexamined Publication No. 6-342024. Another conventional method determines the deterioration of a capacitor by controlling the power distribution of the capacitor to determine the deterioration of the capacitor when the inter-terminal voltage of the capacitor reaches a deterioration reference voltage within a predetermined time from the time at which the inter-terminal voltage reaches a predetermined value. This deterioration determination method is disclosed by Japanese Patent Unexamined Publication No. 2001-297954.
Still another conventional method determines the deterioration of a capacitor by measuring the increase of the temperature at the surface of the capacitor. The above-described deterioration determination method is disclosed by Japanese Patent Unexamined Publication No. 2001-85283. A technique regarding an impedance characteristic of a capacitor is disclosed, for example, by “Denki-Kagaku Capacitor, Kiso, Zairyou, Ouyou (which is written by Brian E. Conway and published by NTS Inc. on Jun. 5, 2001, P. 393 to P. 401).
However, the conventional methods for determining the deterioration of a capacitor may require a circuit section (e.g., measurement signal source, A/D converter) and a signal processing by CPU, thus increasing the cost of the measurement apparatus and complicating the deterioration detection method. The above deterioration determination by the temperature at the surface of the capacitor also may cause a problem in the measurement accuracy.
Furthermore, generally-known conventional methods for determining the deterioration of an electrolysis capacitor may cause a huge amount of accumulated measured data. Thus, a circuit device for determining the deterioration based on the data also may have a high cost and a complicate structure.
Furthermore, when a capacitor in which electrolytic solution is provided between electrode bodies (e.g., electric double layer capacitor) is determined with regards to the deterioration, another method may be considered by which the capacity component and a DC capacitor resistance (hereinafter referred to as DCR) component are measured and the determination is made based on the measurement result. For the measurement of these capacity component and DCR component, a DC voltage method and an AC impedance method are known. The DC voltage method measures these capacity component and DCR component based on the behavior of the DC voltage when the capacitor performs charging and discharging. The AC impedance method applies an AC voltage to the capacitor to derive these capacity component and DCR component from the impedance value.
The DC voltage method provides an accurate measurement result because this method directly measures these capacity component and DCR component from the DC voltage in the charging and discharging but uses a large amount of charge in the capacitor due to the charging and discharging. Thus, the DC voltage method may cause a situation where much power is consumed for the determination of the deterioration. The AC impedance method consumes less power because this method uses the frequency characteristic of the AC voltage. However, the AC impedance method may cause, when the deterioration of the capacitor is promoted, a value that is smaller than that of the DC voltage method. This may cause a situation where this value is mistakenly determined as representing no deterioration, thus lowering the reliability.